It's obvious that the user is unknown in the local recipient as it does not exist on that server! (It does exist on the other server that it should go to)
You can also see that it does not even try to connect to the other server.

Doing a 'dig domain.tld MX' on the troubled system does show the correct server IP.

I'm lost at the moment, and hope that someone here migh know how to start debugging this problem?